With the development of optical systems expected to provide extremely high levels of optical performance, much attention has been directed to the accuracy and precision to which the constituent optical elements are manufactured. Attention has also been directed to the manner in which the optical elements of the system are mounted relative to each other on a frame or in a “barrel.” To increase the optical performance of certain optical systems even further, the barrel is configured to provide a stringently controlled operational environment for the optical elements, such as controlled temperature and pressure. In addition, the barrel or frame is mounted on a machine frame by mounts that damp or attenuate transmission of vibrations from the machine frame to the barrel.
An exemplary technology in which high-performance optical systems are critical is microlithography. Microlithography is essentially a photographic process by which fine patterns, such as micro-circuit patterns, are transferred or otherwise formed on an exposure-sensitive substrate such as a glass, ceramic, or crystalline plate or a semiconductor wafer, for example. In the manner of a photographic process, microlithography utilizes light or other energy beam to expose the pattern image on the substrate.
Since the advent of microlithography, the workhorse wavelength of light for making microlithographic exposures has progressively shortened from the deep-ultraviolet (DUV) range to the vacuum-UV (VUV) range, the latter ranging approximately from 150 to 300 nm. Recently, substantial effort has been directed to the development of a practical microlithography system utilizing “extreme-UV” (EUV) wavelengths in the range of 11 to 15 nm, most typically approximately 13.5 nm. The trend of using progressively shorter wavelengths of exposure light in microlithography is based on the principle that reducing the wavelength of exposure light generally provides better and finer imaging resolution. In other words, a shorter wavelength of exposure light generally can form a pattern on the substrate with better resolution of correspondingly finer pattern elements, compared to a longer wavelength.
Whereas DUV and VUV optical systems can be totally refractive or catadioptric (the latter having both refractive and reflective optical elements), EUV optical systems must be all-reflective (catoptric) because no known materials are suitable for making EUV lenses. In an EUV optical system, the constituent reflective optical elements are generally called “mirrors.” Except for grazing-incidence mirrors, the optical surface of a typical EUV mirror includes a multilayer film to maximize reflectivity to incident EUV light. (The maximum achievable reflectivity is approximately 70%.)
The mirrors of an EUV optical system, such as a projection-optical system, are mounted in a barrel or optical frame using mirror mounts. Mirror mounts are usually configured to hold the mirror in the barrel or frame accurately without over-constraining the mirror. The mirror mounts usually are also configured, at least in a passive manner, to attenuate transmission of disturbances and vibrations from the barrel to the mirror. Also, the barrel or optical frame is usually attached to other system structure using vibration-isolation mounts that inhibit transmission of external disturbances to the barrel or frame from other parts of the machine or from the floor in which the machine is placed. In some systems the mounts of at least one mirror of the EUV optical system are “active-isolation” types in which the mirror is provided with slight controlled movability relative to the barrel or optical frame as required to maintain a desired position of the mirror despite movements of the barrel. The movability is controlled by feedback to compensate mirror position, that otherwise would be affected by movements of the barrel or optical frame, and thus improve image quality. A mirror or other element having such mounts is called an “active-isolation-mounted” element.
Conventionally, the active-isolation mounts for a mirror in an EUV optical system comprises mirror servos of which the actuators are piezoelectric transducers (abbreviated herein as “PZTs”). (Note that “PZT” in the relevant art also denotes lead zirconate titanate, a piezoelectric ceramic material. Lead zirconate titanate is a common material, but not the only material, for making PZTs. Hence, not all PZTs are made of lead zirconate titanate.) Piezoelectric materials are distorted by application thereto of an electric field. Piezoelectric ceramic materials used in PZTs are electrically poled during manufacture by applying a large electric field during high-temperature annealing of the material. The PZT is placed, in contact with the mirror, between the mirror and a rigid and stable support. During use of the PZT, application of an electric field along the polarization direction forces the ceramic material to expand in directions perpendicular to the electric field. Conversely, application of an electric field in a direction opposite the polarization direction forces the ceramic material to contract in directions perpendicular to the electric field. This expansion or contraction imposes a local stress on the mirror. For general information regarding an optical mount including PZT actuators, see U.S. Pat. No. 6,317,195, particularly FIG. 1 and associated text, incorporated herein by reference. PZTs are compact, light-weight, and rugged, and operate in a consistent and controllable manner. The amounts of mirror motion provided by the PZTs are very small but nevertheless adequate.
Unfortunately, it is difficult for PZT actuators used as mirror servos to meet the stringent specifications of an EUV microlithography system for reasons such as the following: (1) PZT elements have high stiffness. Since they directly contact either the mirror or other structure in contact with the mirror, vibrations are readily transmitted via the PZT elements from the barrel or optical frame to the mirror. (2) PZT elements are continuously energized to hold their positions, which can produce vibrations and transmit them to the mirror. (3) PZT elements in combination with structure in which they are in contact tend to produce additional vibrational modes. For example, a PZT servo can interact with the mirror and produce additional vibrational modes of the mirror. (4) Disturbances of the barrel or frame are mainly of relatively high frequency. Mirror servos based on PZT elements have relatively low bandwidth, which prevents the PZT-actuated mirror from catching up with high-frequency movements of the lens barrel or optical frame. Since the ability to prevent transmission of disturbances is currently evaluated based on the relative distance between the mirror and the lens barrel, this results in unacceptable rejection of disturbances propagating from the barrel or frame to the mirror.
Therefore, there is a need for improved servos for use in active-isolation mounts for optical elements, such as but not limited to reflective optical elements (“mirrors”) used in an EUV optical system or other high-performance catadioptric or catoptric optical system.